The infrared sensitive materials Hg.sub.1-x Cd.sub.x Te and H.sub.1-x Zn.sub.x Te are used in the production of photovoltaic mosaic focal plane arrays since their fundamental absorption edges can be adjusted to coincide with atmospheric window wavelengths (3-5 micron and 8-12 micron) by a suitable choice of the composition x. Thin layers of these materials may be grown epitaxially upon CdTe or upon related alternative substrates via liquid phase epitaxy at atmospheric pressure from a tellurium-rich molten solution (ref.Harman). Such single-film and multi-film crystalline layers are advantageous for the production of monolithic and hybrid Hg.sub.1-x Cd.sub.x Te and Hg.sub.1-x Zn.sub.x Te detectors and CCD's. For cost-effective focal plane array production, high quality epitaxial layers of these substances grown on a large area (greater than 675 mm.sup.2) single-crystal substrates are demanded; growth processes must inherently produce such films with high reliability, reproducibility, yield and throughput.
Generally, graphite growth susceptors of a wide variety of designs (Ref: Bowers, et al) are employed for such LPE growths. As the demand for the reproducible high-rate manufacture of large area II-VI single and multifilm epilayers increases, the growth apparatus subsequently becomes larger and more complex.
Having many desirable physical, chemical and thermal properties, graphite is a popular and widely employed LPE boat material.
For the advanced application to II-VI compound semiconductor epitaxy though, raw graphite has certain drawbacks, which are apt to lead to a reduced throughput of epitaxial material and in an undesirably high rate of graphite component rejection.
Owing to its viscosity and surface tension characteristics, molten tellurium-rich growth solutions tend to physically wick between mated, slidable graphite growth components. This phenomenon becomes complicated when growth fixtures are more and more complex, and when growth solutions become larger and larger in order to accommodate sizable area substrate materials. Upon cooling, the apparatus following LPE growth, physical adhesion of solidified melt material onto graphite surfaces frequently occurs. This phenomenon is aggravated by graphite having a relatively porous surface.
Oftentimes, the wafer physically adheres to the graphite susceptor, risking damage upon removal. The excessive need for graphite repurification and handling is likely to lower the purity of subsequent products and risks damage to delicate boat components. Overall reliability, throughput and yield of the products suffer. In addition, since II-VI epitaxial growths employing mercury require that evaporative mercury loss be controlled and prevented, it is essential that all surfaces of graphite growth fixtures mate with the highest machinable perfection. Graphite surfaces must be smooth and free of defects in order to additionally encourage thorough melt decantation following epitaxial growth. Being a relatively soft material, graphite surfaces are easily damaged and abraded during repetitive growth processes. Carbon particles may be shed because of such poor abrasion resistance. To overcome these drawbacks while maintaining the overall advantageous characteristics of graphite, encapsulants or coatings are typically applied to graphite LPE components.
Some of the conventional coatings are:
1. Vapor deposited pyrolytic carbon films:
For applications including bulk crystal growth, zone refining and liquid phase epitaxy, this coating was developed in order to eliminate the effect of carbon contamination and excessive graphite dusting. It is stable in oxidizing atmospheres at up to 450.degree. C. and is not wetted by most molten metal and molten salt solutions . Unfortunately, the micromorphology of this coating often contains miniscule imbedded particulates, and therefore the close fit between tightly mated graphite components is reduced. Internal corners of machined graphite products become slightly filled with coating material, lowering the precision and accuracy of dimensions. These pyrolytic coatings also have poor abrasion resistance and a tendency to peel from the underlying graphite.
2. Glassy carbon coating:
This coating is mechanically applied and subsequently baked. Generally, micromorphology is quite poor and internally machined corners are filled with coating.
3. Vapor deposited boron nitride:
Although having suitable chemical, physical and thermal properties, this encapsulation is quite non-uniform in thickness and has an inadequately rough surface morphology.
4. Evaporated coatings of SiO.sub.x :
This coating technique has been employed where excessive wetting by molten metallic solutions in metallurgical applications is to be eliminated. As experienced by Hass, et al.. the direct evaporation of silicon dioxide SiO.sub.2 is difficult because of the extreme temperatures involved. SiO.sub.2 films tend to have doubtful protective qualities because of loose structure. Evaporated films of SiO.sub.x (x is oftentimes greater than 1 since it is difficult to avoid the incorporation of extra oxygen into the relatively unstable Si--O bond when evaporation is performed under a non-ideal vacuum)tend not to have adequate adhesion to graphite components if thick (greater than 10,000 Angstroms). If thinner, such coatings generally have a poor abrasion resistance. Such thin films (5000-9000 Angstroms) easily abrade and create dust when tightly contacted graphite components move against one another, causing limited graphite lifetime and a need to reapply the SiO.sub.x coating often. SiO.sub.x particulates may interfere with the purity of the process. Furthermore, if internal cavities and machined recesses need to be uniformly coated, evaporation techniques frequently coat in a non-homogeneous manner since it is a "line-of-sight" deposition technology. Purity also is oftentimes threatened since, possibly, impurities outgassing from the evaporationsource heating elements, may become incorporated into the evaporated films.